Flow cells or flow batteries are electrochemical cells that do not have solid electrodes but instead have liquid active materials: redox active materials that are liquid in both reduced and oxidized states. Because there are no solid electrodes to be regenerated via charging, a flow battery can be recharged by draining the discharged liquid active material and refueling with charged liquid active material. This capability to be quickly recharged by refueling makes the use of flow batteries a potentially valuable approach to powering electrical systems that are in near constant use such that extended recharging times would be unacceptable, for example electrically powered municipal transportation vehicles.
However, because flow batteries lack a pool of reduced active material in the form of a solid anode and a sink for oxidized active material in the form of a solid cathode, they typically suffer from low energy density. This low energy density results in a need for frequent refueling and thus largely offsets the value derived from the capability of rapid recharging by refueling.
One approach to offsetting the low energy density of flow batteries is to equip them with large reservoirs of active material. This approach may be impractical for mobile systems or systems which otherwise require modest size, however.